1. Field
Embodiments of the present invention relate to continuous dense phase pneumatic convey systems. More particularly, embodiments of the present invention relate to a low pressure continuous dense phase convey system that employs a non-critical air control system so as to achieve relatively low energy consumption while maintaining minimal particulate degradation.
2. Related Art
A low pressure continuous dense phase (“LPCDP” or “CDP”) convey system uses a low pressure air supply to drive a low effective air velocity (approximately 400-1800 feet per minute (“FPM”)) pneumatic convey system. The CDP convey system employs an airlock to feed material into a convey line at a feedpoint of the convey system. To allow the CDP system to operate effectively, an air control system is employed for controlling a rate of air flow through the convey system as the conveying pressure repeatedly increases and decreases during conveying operation. In general, a desired rate of air flow through the convey system is calculated by adding a predicted amount of leakage air to the desired amount of conveying air that passes through the convey line.
To obtain the desired amount of conveying air through the convey line, prior art convey systems employ a critical air flow control system. In such a system, the air pressure produced by an air source, such as a blower, is significantly greater than the air pressure needed to convey the material. In one prior art system and method, a blower for producing an air flow is positioned upstream of the feedpoint. Air produced by the blower is supplied across an infinite position sonic nozzle (“IPSN” or the “nozzle”). The nozzle acts as a control valve for controlling or otherwise metering the amount of supply pressure P1 is nodpoint of the convey system.
In a critical air flow control system, the air supplied from the blower, i.e., the supply pressure, P1, and to the control valve, i.e., the nozzle, is held constant. The air flow exiting the control valve can be modeled, such that if the supply pressure, P1, communicated to the control valve is held constant, then the air flow across the control valve is predictable using critical flow calculations. Thus, the air flow exiting the control valve is also predictable by valve position only and is unaffected by a downstream convey pressure, P2. The relationship of P1 to P2 in the critical air flow regime is the following:
      (                  P        ⁢                                  ⁢        1            +              P        a              )    ≥            (                        P          ⁢                                          ⁢          2                +                  P          a                    )        Efficiency  where Pa is atmospheric pressure, and “Efficiency” is a percentage value representing the efficiency of the control valve, which is known. The Efficiency of a valve is usually referred to as the value to which P2 can rise in a critical air flow system before the rate of air flow is decreased as a result. For example, in an inefficient valve, P2 can only rise to 50%-60% of P1, whereas in highly efficient valve, P2 can rise to approximately 80%-85% of P1.
To insure that the supply pressure, P1, to the control valve is constant, a mechanical relief valve is positioned downstream of the blower and upstream of the control valve. The mechanical relief valve is used to vent any excess air to atmosphere that is generated by the blower and that is not used by the control valve. Thus, the mechanical relief valve assists in maintaining the constant supply pressure, P1, to an inlet of the control valve.
In other prior art systems, an IPSN or control valve is not employed between the blower and the feedpoint of the convey system. Instead, once the air is discharged from the blower, it is provided to the feedpoint of the convey system with no further modulation or control. Modulation is instead performed on the blower inlet, where throttling changes the blower performance. The blower operates at a constant speed and is throttled as necessary. Because the convey system pressure is constantly changing, the blower pressure is constantly throttled, resulting in inefficient and unnecessary air production by the blower. During conveying operation, the blower fluctuates between being throttled a small amount or not at all to being throttled a large amount.
In both of the above-described prior art systems, the differential pressure created is significantly greater than the air supply pressure actually required to be delivered to the feedpoint of the convey system. For example, in the first prior art system described above, the supply pressure produced by the blower remains critical at all times to insure the flow across the control valve is predictable by the above critical flow equation. As such, it is common for a ratio of the supply pressure, P1, to the convey pressure, P2, to be 1.6:1-2:1. This results in the power required to drive the air control system being 1.6×-2× greater than the energy required to move the material. Additionally, energy is lost to controlling the air control system accurately. In systems where the unused air is vented to atmosphere, a varying volume of air is unnecessarily compressed, thus creating further inefficiency. In the second prior art system described above, the unnecessary throttling of the blower inlet also produces inefficiency in energy consumption.
Employing a critical air flow regime is relatively computationally and structurally simple; it is simpler to vent or otherwise reduce an excess air supply pressure to the feedpoint than monitor and responsively control the air supply pressure across the control valve. Moreover, because P1 is held constant and because it is critical, the rate of air flow exiting the control valve is independent of P2, i.e., the air flow to the convey system is a function of valve position only. If the air control system relies on a constant air supply pressure to the control valve, then the above critical air flow equations allow for a relatively simple computation of the convey pressure, P2, needed for the convey system. Prior art air control systems thus essentially overcompensate for the air supply pressure required to operate the convey system properly with the negative consequence of increased energy consumption.
Accordingly, there is a need for an improved air control system for a CDP convey system. In particular, there is a need for an air control system that is more efficient, less costly, and that does not rely on critical air flow to achieve the desired amount of conveying air to the convey system.